California’s coast is dominated by sedimentary rocks and worn, wave-polished stones. Many coastal bluffs are made of sandstone, shale, and conglomerate deposited in ancient marine environments​. These rocks often break down into smaller pieces that wash onto beaches.

On ocean beaches and bays, you can find smooth pebbles and cobbles of various colors – common materials include chert, quartzite, and basalt that have been rounded by the surf. Keep in mind that some picturesque beach stones (like the well-known white and black pebbles of Big Sur or jade pieces in Monterey County) may be in protected areas, so check local rules.

Coastal rivers and creek outlets also deposit interesting rocks at their mouths. If you’re near sea cliffs, you might see chunks of the local bedrock (sandstone or even granite in some areas like San Diego County) that have fallen – these can be collected if accessible and permitted. Also, note that Central Coast vs. Southern Coast differ a bit: the north/central coast has more dark greywacke sandstone and greenish serpentinite in places, whereas southern coastal areas can have colorful volcanic rocks from nearby hills.

Beaches are easy to access and often have an abundance of rounded stones great for garden borders or mosaics, but always double-check if removing them is allowed (some beaches permit taking a small bucket for personal use, others forbid it).

The deserts of California (such as the Mojave and Colorado deserts) are a paradise for rockhounds. In these arid areas you’ll encounter a lot of igneous and metamorphic rocks exposed on the surface​. Common finds include volcanic rocks like basalt and rhyolite, shiny black obsidian in certain volcanic fields, and an array of quartz minerals.

The Mojave Desert in particular is known for jaspers and agates (often found as nodules or scattered fragments in alluvial fans and washes). For example, the Cadiz area and Lavic Lake (in the Mojave Trails National Monument) are famous for red and yellow jaspers, and the Hauser Geode Beds (near the Colorado Desert border) yield geodes and nodules of quartz and chalcedony. Desert rocks often have a dark “desert varnish” (a brownish-black patina) from long exposure – this can add a beautiful weathered look in landscaping. You might also find petrified wood or marine fossils in some desert localities (like in the Cady Mountains or Anza-Borrego desert), but remember that vertebrate fossils and artifacts are off-limits without permits​.

The desert is vast and much of it is BLM land where casual collecting is allowed. Look for dry lakebeds, washes, and the bases of rocky outcrops – these tend to have abundant loose material. Always carry plenty of water and watch for extreme heat while desert collecting. The reward can be spectacularly colored stones that bring Southwest flair to your garden.

California’s mountains (Sierra Nevada, Cascades, Coast Ranges, etc.) offer completely different rock types. Granite and other granitic rocks are very common in the Sierra Nevada and parts of the southern California ranges​ – think of the big grey granite domes of Yosemite.

Pieces of granite, diorite, and related rocks are often found at the foot of cliffs or in stream beds in these areas. They make excellent rugged-looking landscaping boulders or smaller border rocks. The foothills of the Sierra and Klamath Mountains also have a lot of metamorphic rocks (schist, slate, marble, quartzite) thanks to ancient uplift and subduction processes​. One famous rock from the Sierra foothills is serpentine (serpentinite), the state rock of California, which is a greenish metamorphic rock often seen in outcrops along highways​.

Serpentine’s green color and smooth texture can be quite attractive, though it can be brittle. In volcanic mountain areas (like Mt. Shasta or Lassen region), you may find porous lava rocks, pumice, and volcanic tuff. In the Coast Ranges (e.g., around the Bay Area and northward), expect lots of layered sedimentary rock (sandstone, shale) and some metamorphics; these can provide flat stones that work well for stepping paths or dry walls. When collecting in mountains, pay attention to road cuts and natural cliffs – these often expose fresh rock.

Be cautious on steep slopes, and be aware that higher elevations might be on National Forest land (allowed) or possibly within wilderness or park boundaries (need to verify rules). The variety is huge – you could collect everything from granite boulders to pretty white quartz veins and even small amounts of gold-bearing ore in some historic mining areas (though don’t expect to strike it rich while gathering landscape rocks!).

Rivers and creek corridors are fantastic for finding smooth, rounded rocks of all sizes. Over years, water flow tumbles rocks against each other, creating the classic “river rock” that landscapers love to use.

In California, many rivers originate in mountains and carry down granite, quartz, and other hard rocks, depositing them in the valleys. For instance, rivers flowing out of the Sierra Nevada (American River, Merced, Feather, etc.) are loaded with cobbles of granite, quartzite, and sometimes volcanic rocks, polished to a pleasant roundness. These are ideal for dry creek bed features or as natural edging.

Stream gravel bars often contain a mix of rock types from all upstream sources – you might pick up milky quartz, greenstone, even the occasional jade pebble on certain rivers.

Tip: look for point bars (the inner bend of a river) where flow slows and rocks accumulate. Late summer or fall, when water is low, is the best time to collect from rivers.

Also, some state recreation areas allow rockhounding, specifically on “beaches or gravel bars subject to annual flooding” in streams​, which implies these are recognized as renewable sources (new rocks wash in each year). Still, avoid altering the streambed significantly; take a few rocks from many spots rather than all the rocks from one spot to minimize impact.

In smaller creeks, especially in gold country, you might stumble on interesting heavy rocks like pieces of jadeite, garnets, or even small gold nuggets trapped in gravel (remember that gold prospecting has its own rules, so check those if that’s your aim).

Along large river banks, be mindful of private property (many riverfront parcels are privately owned). Also, never wade into a river during high flows or without assessing safety. But overall, rivers are among the best and most accessible places to gather beautiful stones that have already been naturally “weathered” for you.